Due to the development of electronic technology, or the growing interest in environmental technology in recent years, various electrochemical devices are used. In particular, requests for energy saving, and expectations for electrochemical devices that can contribute to the above have been increasing more and more.
A lithium ion secondary battery which is a representative example of a power storage device, and also a representative example of a nonaqueous electrolyte battery, has been conventionally used mainly as a power source for a small device, but in recent years, it has drawn attention as a power source for a hybrid vehicle or an electric vehicle.
With respect to a lithium ion secondary battery, increase in energy density is advancing along with enhancement of device performances, and reliability is becoming more important. In particular, in the case of a medium- or large-sized lithium-ion secondary battery such as an automotive onboard power source, it is essential to ensure higher reliability than in the case of small devices.
In a lithium ion secondary battery, a lithium (Li) ion moves between a positive electrode and a negative electrode to carry out charge and discharge. However, it has been known that, if a trace amount of metal ions other than a Li ion is present in the battery, it may deposit on the negative electrode surface to cause reduction in the battery life, or the deposited metal may break a separator and reach the positive electrode to cause a short circuit or deterioration of the safety. Such metal ions are originated from impurities in a battery constituent material, and additionally some are derived by dissolution of a metal included in a positive electrode active material into a nonaqueous electrolyte as the result of a side reaction in the battery (for example, a decomposition reaction of the nonaqueous electrolyte). Further, such metal dissolution is more remarkable when the battery is exposed to a high temperature. For this reason, investigations have been made on a scavenger that captures impurities such as metal ions (see, for example, Patent Literature 1 and 2).
Further, in recent years, a lithium ion battery is required to be charged at a high voltage of 4.35 V or more in order to increase further the energy density. Particularly when it is used as a power source for an electric vehicle, the service environmental temperature of the lithium ion battery may occasionally be 50° C. or even higher. It has been known that, in such a high voltage and high temperature environment, metal dissolution from the positive electrode active material is accelerated and the reduction in battery life is further conspicuous. Therefore, a lithium ion battery needs to maintain a capacity even when charging and discharging are repeated in the environment.